Method and apparatus for dynamic current distribution control during electroplating

ABSTRACT

A method of electroplating a metal on a cathodically biased wafer substrate employs an electroplating apparatus having a main anode, an auxiliary electrode and an ionic current collimator, where the ionic current collimator is configured to direct ionic current that is generated by the main anode from a periphery of a plating vessel to its center. During electroplating, in a first electroplating stage (e.g., when terminal effect is pronounced), the metal is plated onto the wafer while the auxiliary electrode is cathodically biased; and in a second electroplating stage (e.g., when terminal effect subsides), the metal is plated onto the wafer while the auxiliary electrode is anodically biased. In some embodiments the auxiliary electrode is located between the ionic current collimator and the wafer, and is configured to redistribute ionic current that has passed through the collimator. The method is useful for improving the uniformity of electroplating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of and claims priority to U.S.application Ser. No. 14/067,616 filed on Oct. 30, 2013, titled “Methodand Apparatus for Dynamic Current Distribution Control duringElectroplating” naming Zhian He as the inventor, which claims benefit ofprior U.S. Provisional Application No. 61/730,285 filed on Nov. 27,2012, titled “Method and Apparatus for dynamic Current DistributionControl during Electroplating” naming Zhian He as the inventor, whichare herein incorporated by reference in their entirety and for allpurposes.

FIELD OF THE INVENTION

The present invention pertains to methods and apparatuses forelectroplating. Specifically, the invention pertains to electroplatingtools used for electrodeposition of metals in semiconductor processing.

BACKGROUND OF THE INVENTION

The transition from aluminum to copper in integrated circuit (IC)fabrication required a change in process “architecture” (to damasceneand dual-damascene) as well as a whole new set of process technologies.One process step used in producing copper damascene circuits is theformation of a “seed-” or “strike-” layer, which is then used as a baselayer onto which copper is electroplated (electrofill). The seed layercarries the electrical plating current from the edge region of the wafer(where electrical contact is made) to all trench and via structureslocated across the wafer surface. The seed film is typically a thinconductive copper layer. It is separated from the insulating silicondioxide or other dielectric by a barrier layer. The use of thin seedlayers having dual barrier-seed function (e.g. alloys of copper, orother metals, such as ruthenium and tantalum), has also beeninvestigated.

As semiconductor industry advances, technology nodes are moving towardsvery thin and resistive seed regime for electrochemical fill. It becomesa very challenging problem to achieve uniform initial plating across thewafer with such resistive seed layers. To effectively plate a largesurface area, the plating tool makes electrical contact to theconductive seed only in the edge region of the wafer substrate. There isno direct contact made to the central region of the substrate. Hence,for highly resistive seed layers, the potential at the edge of the layeris significantly greater than at the central region of the layer.Without appropriate means of resistance and voltage compensation, thislarge edge-to-center voltage drop could lead to an extremely non-uniformplating thickness distribution, primarily characterized by thickerplating at the wafer edge. This effect is known as terminal effect.

The non-uniform plating thickness will be even more pronounced as theindustry transitions from 300 mm wafer to 450 mm wafer.

SUMMARY

The difficulty in controlling the terminal effect is further exacerbatedby the fact that it is very pronounced in the beginning ofelectroplating when the seed layer on the wafer is most resistive, butis rapidly diminishing during the course of electroplating. Aselectroplating proceeds, the plated layer becomes thicker and moreconductive, thereby reducing the terminal effect. Therefore, during asingle electroplating process, very different ionic current environmentsshould be created in the plating apparatus in order to compensate forthe terminal effect in the beginning of the process, and to continueelectroplating after the terminal effect has subsided.

The needs for controllable electroplating on resistive seed layers areaddressed herein by providing an apparatus and a method forelectroplating that make use of a focus ring (also referred to as anionic current collimator) positioned in the proximity of an anode, andan auxiliary electrode with flexibly adjustable electricalcharacteristics. The ionic current collimator provides a resistivecorrection for the terminal effect by restricting the ionic current atthe periphery, and by directing the ionic current to the centralportions of the wafer substrate. However, the use of ionic currentcollimator alone would have resulted in unnecessarily center-thickplating. An auxiliary electrode residing around the ionic currentcollimator corrects this problem, by diverting the ionic currentprovided by the collimator away from the center, and modifying it tomake it more uniform. In some embodiments, the auxiliary electrode isconfigured to serve as an auxiliary cathode in the beginning of theelectroplating process, when the terminal effect is most pronounced, andis also configured to serve as an auxiliary anode (or even as a mainanode) later in the process. The auxiliary electrode, in someembodiments, is configured to be able to accept a large amount ofcurrent (e.g., at least about 200% of the current supplied to the waferin the beginning of electroplating, such as at least about 400% of thecurrent supplied to the wafer in the beginning of electroplating), andis characterized by an unusual location in the electroplating apparatus.The auxiliary electrode, in some embodiments at least partially residesover the main anode, and has a non-zero footprint on the anode. In someembodiments, the auxiliary electrode is also characterized by a largesurface area, which allows it to accept large currents without buildingexcessively high current densities. For example, the auxiliary electrodein some embodiments is capable of accepting currents of between about10-75 A, such as between about 20-50 A.

In one aspect, an apparatus for electroplating metal on a wafersubstrate is provided. The apparatus comprises: (a) a plating vesselconfigured for holding an electroplating solution therein; (b) a waferholder configured for holding the wafer substrate in position duringelectroplating, the wafer holder having one or more electrical powercontacts arranged to contact an edge of the substrate and provideelectrical current to the substrate during electroplating, wherein theapparatus is configured for cathodically biasing the wafer substrateduring electroplating; (c) an anode (also referred to as “main anode”)residing in the plating vessel and configured to be anodically biasedduring at least a portion of electroplating; (d) an ionic currentcollimator proximate the anode, wherein the ionic current collimator isa non-conductive member configured to direct the ionic current from theanode generally from the periphery to the center of the plating vessel;and (e) an auxiliary electrode configured to be both cathodically andanodically biased during electroplating.

In another aspect, a method for electroplating a layer of metal on awafer substrate is provided. In some embodiments, the method involves(a) providing the wafer substrate to an electroplating apparatus havinga wafer holder, and a plating vessel containing a main anode, anauxiliary electrode, and an ionic current collimator, wherein the ioniccurrent collimator is configured to direct ionic current from theperiphery to the center of the plating vessel; (b) in a firstelectroplating stage, electroplating metal onto the wafer substratewhile cathodically biasing the auxiliary electrode; and (c) in a secondelectroplating stage, electroplating metal onto the wafer substratewhile anodically biasing the auxiliary electrode.

In another aspect, a non-transitory computer readable medium comprisinginstructions for control of an electroplating apparatus is provided. Theprogram instructions will include code for performing the methodsprovided herein, such as (a) in a first electroplating stage,electroplating metal onto the wafer substrate while cathodically biasingthe auxiliary electrode; and (b) in a second electroplating stage,electroplating metal onto the wafer substrate while anodically biasingthe auxiliary electrode.

These and other features and advantages of the present invention will bedescribed in more detail below with reference to the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional view of an electroplatingapparatus in accordance with an embodiment presented herein.

FIG. 1B is a schematic cross-sectional view of an electroplatingapparatus in accordance with another embodiment presented herein.

FIG. 1C is a schematic cross-sectional view of an electroplatingapparatus in accordance with another embodiment presented herein.

FIG. 2A is a schematic cross-sectional view of an ionic currentcollimator residing over an anode.

FIG. 2B is a schematic top view of an ionic current collimator.

FIG. 3A is a schematic cross-sectional view of an auxiliary electrode,an ionic current collimator and an anode in accordance with anembodiment presented herein.

FIG. 3B is a schematic top view of an auxiliary electrode in accordancewith an embodiment presented herein.

FIG. 4 is a schematic diagram illustrating electrical connectivitybetween the controller, power supply, and components of the plating cellin accordance with an embodiment provided herein.

FIG. 5A is a process flow diagram for an electroplating method inaccordance with an embodiment provided herein.

FIG. 5B is an example of an algorithm for determining and usingcontroller instructions in accordance with an embodiment providedherein.

FIG. 6A shows a computational modeling result illustrating ionic currentdistribution in the electroplating apparatus during the first stage ofplating, in accordance with an embodiment provided herein.

FIG. 6B shows a computational modeling result illustrating ionic currentdistribution in the electroplating apparatus during the second stage ofplating, in accordance with one embodiment provided herein.

FIG. 7 is a plot illustrating preferred current levels for differentcomponents of the electroplating cell, as a function of seed layer sheetresistance, in accordance with one example presented herein.

FIG. 8 is a plot illustrating preferred current levels for an auxiliaryelectrode as a function of electroplating time.

FIG. 9 is an illustration of instantaneous current distribution on thewafer substrate at various stages of the plating.

DETAILED DESCRIPTION

The methods and apparatus provided herein are useful for electroplatinga variety of metals including but not limited to copper and its alloyson semiconductor substrates having one or more recessed features (e.g.,trenches and vias). The methods and apparatus are useful forelectroplating on 300 mm and, particularly, on 450 mm semiconductorwafers and on resistive seed layers. For example, the apparatus andmethods, can be used, in some embodiments, for electroplating on seedlayers having seed sheet resistance of up to about 50 Ohm/sq. (inclusiveof this number), e.g. with sheet resistance of between about 10-50Ohm/sq., such as between about 20-40 Ohm/sq. Examples of substrates thatcan be processed by provided methods include, without limitation, a 300mm wafer having a copper seed layer having a thickness of between about10-2000 A, or a 450 mm wafer having a copper seed layer having athickness of between about 20-2000 A. In some embodiments the initialcopper seed layer thickness is between about 10-100 A, e.g., betweenabout 10-50 A.

The methods and apparatus described herein can be used to provide platedlayers having excellent center-to-edge uniformity due to their highcapability for controlling ionic current environment within theelectroplating bath. While in many implementations uniform plating isdesired, in some embodiments, when center-thick or edge-thick plating isrequired, the apparatus can be configured to control ionic currentdistribution such as to introduce the desired non-uniformity.

In one aspect, an apparatus for electroplating is provided. Theapparatus comprises: (a) a plating vessel configured for holding anelectroplating solution therein; (b) a wafer holder configured forholding the wafer substrate in position during electroplating, the waferholder having one or more electrical power contacts arranged to contactan edge of the substrate and provide electrical current to the substrateduring electroplating, wherein the apparatus is configured forcathodically biasing the wafer substrate during electroplating; (c) ananode residing in the plating vessel (also known as the main anode); (d)an ionic current collimator proximate the anode, wherein the ioniccurrent collimator is a non-conductive member configured to direct theionic current from the anode generally from the periphery to the centerof the plating vessel; and (e) an auxiliary electrode configured to beboth cathodically and anodically biased during electroplating.

The ionic current collimator is made of dielectric material (e.g.,plastic) that is not permeable to electrolyte. Examples of suitablematerials include polycarbonate, polyethylene, polypropylene,polyvinylidene difluoride (PVDF), polytetrafluoroethylene, andpolysulphone. In some embodiments the ionic current collimator comprisestwo portions: (i) the central portion which is generally in the form ofan open cylinder extending in the direction that is perpendicular to theplane of the wafer substrate (referring to the plane of the platingsurface of the substrate), and, typically, co-centered with the centerof the wafer substrate and the center of the anode, where the openingsof the cylinder provide a route for the ionic current; and (ii) thecurrent restricting portion which is connected to the cylindricalportion, e.g., at the end of the cylindrical portion that is proximatethe anode, and which is generally parallel to the plane of the wafersubstrate. The current restricting portion typically extends to thesidewalls of the plating vessel and is secured (e.g., attached) to thesewalls such that the ionic current collimator is held in place, and suchthat the ionic current from the anode would not be able to escape at theperiphery of the plating vessel. Thus, the ionic current collimator candirect substantially all of the current from the main anode through itscentral cylindrical opening generally in the direction of the center ofthe wafer. The ionic current collimator, in some embodiments, does notcontact the anode and is spaced apart from the anode by at least about15% of the wafer radius (e.g., by at least about 40 mm), for example byaround 60 mm. The spacing from the anode determines the amount of anodeutilization, and also impacts the thickness or current density profile.For example, if the collimator is too close to the anode, the anodeutilization may be relatively small.

In some embodiments, the ionic current collimator is stationary, anddoes not move during electroplating.

In other embodiments, the apparatus is configured to move the ioniccurrent collimator along an axis that is perpendicular to the plane ofthe wafer substrate. For example, the collimator can be moved closer tothe wafer when more ionic current at the wafer center is desired, andcan be moved away from the wafer when less center-focused current isneeded. In some implementations, the apparatus includes a mechanismconfigured for moving the ionic current collimator. For example, thecollimator can be moved with a bellows-type mechanism.

The auxiliary electrode is located between the anode and the wafersubstrate or the wafer substrate holder (referring to a position on anaxis that is perpendicular to the wafer surface), and further away fromthe anode than the current-restricting portion of the collimator. Insome embodiments, the current-restricting portion of the collimatorserves as a convenient platform on which the auxiliary electrode residesin the electroplating apparatus. The auxiliary electrode is electricallyconnected to a power supply and can be biased negatively or positively,as desired. When biased negatively, the auxiliary electrode serves as anauxiliary cathode and is capable of diverting ionic current towarditself, thereby reducing the current experienced by the wafer substrate,and redistributing the center-focused current exiting the cylindricalportion of the collimator. When biased positively, the auxiliaryelectrode serves as an anode and is capable of providing additionalionic current to the wafer. The auxiliary electrode is typically made ofthe same material that is being electroplated. For example, when copperis electrodeposited on the wafer substrate, a copper auxiliary electrodeis used, and serves as a source of copper ions when the auxiliaryelectrode serves as an anode. In some embodiments, the auxiliaryelectrode has a core made of any suitable metal and has a coating of themetal that is being plated (e.g., copper coating). The coating may bepre-made, or can be generated during initial stages of electroplating,when the auxiliary electrode serves as a cathode (due toelectrodeposition on the cathodically biased auxiliary electrode).

Other important characteristics of the auxiliary electrode are itsunusual location and its size. In some embodiments, the auxiliaryelectrode is located not beyond the circumference of the anode, butrelatively closer to the center of the plating bath, and around theopening of the current collimator. In such stacked configuration,utilization of the surfaces for both the main anode and the auxiliaryelectrode would be greater compared to an arrangement in which the anodeand the auxiliary electrode reside in the same plane. The ability toutilize greater surface areas, allows the use of high currents for boththe main anode and the auxiliary electrode without buildingunnecessarily high current densities on these electrodes. This is asignificant advantage, because plating on resistive seed layers oftencalls for the use of very high currents.

In some embodiments, the footprint of the auxiliary electrode projectedonto the anode is at least about 40% of the total anode area, e.g.,between about 60 and 80% of the total anode area. Such position of theauxiliary electrode allows for efficient use of the auxiliary electrodeboth in an anode mode and for redistribution of central current from thecollimator. Further, in some embodiments the auxiliary electrode has alarge surface area. When the surface area is large, the auxiliaryelectrode can accept a very high current (as often needed to compensatefor the terminal effect caused by highly resistive seed layers), withoutbuilding undesirably high current densities. In some embodiments, theworking surface area of the auxiliary electrode (the area that is incontact with electrolyte) is at least about 600 cm², such as betweenabout 900 cm² and 1200 cm². In some embodiments, the auxiliary electrodehas a generally toroidal shape, having a thickness (difference betweenouter and inner radius) that is at least about 20 mm, such as betweenabout 20 mm and 80 mm. For example, the difference between the outer andinner radius, in some embodiments is at least about 5% of the waferradius, such as between about 8-40% of the wafer radius.

In some embodiments the auxiliary electrode is stationary. In otherembodiments the auxiliary electrode is configured to be movable along anaxis that is perpendicular to the plane of the wafer substrate. Theauxiliary electrode can be moved together with a movable ionic currentcollimator, or separately from the collimator. For example, theauxiliary electrode in an anode mode can be moved closer to the wafer,in order to provide more plating current to the center of the wafer. Insome embodiments, the electroplating apparatus includes a mechanismconfigured for moving the auxiliary electrode (e.g., a bellows-likemechanism).

In addition to the ionic current collimator and the auxiliary electrode,the electroplating apparatus may further include additional elementsthat are useful for mitigating the terminal effect. In some embodimentsthe apparatus further includes an ionically resistive element havingelectrolyte-permeable pores or holes, where the element resides in closeproximity of the wafer substrate (e.g., within about 5 mm of platablesurface of the wafer). The ionically resistive ionically permeableelement is useful for improving plating uniformity on thin resistiveseed layers. The ionically resistive ionically permeable elementpresents a uniform current density in the proximity of the wafer cathodeand therefore serves as a virtual anode. Accordingly, the ionicallyresistive ionically permeable element is also referred to as ahigh-resistance virtual anode (HRVA)

In certain embodiments, the HRVA is located in close proximity to thewafer. In certain embodiments, the HRVA contains a plurality ofthrough-holes that are isolated from each other and do not forminterconnecting channels within the body of HRVA. Such through-holeswill be referred to as 1-D through-holes because they extend in onedimension, typically, but not necessarily, normal to the plated surfaceof the wafer. These through-holes are distinct from three-dimensionalporous networks, where the channels extend in three dimensions and forminterconnecting pore structures. An example of a HRVA is a disk made ofan ionically resistive material, such as polycarbonate, polyethylene,polypropylene, polyvinylidene difluoride (PVDF),polytetrafluoroethylene, polysulphone and the like, having between about6,000-12,000 1-D through-holes. In other embodiments, the HRVA is aporous structure in which at least some of the pores are interconnectedand therefore allow some two- or three-dimensional movement ofelectrolyte therein. The disk, in many embodiments, is substantiallycoextensive with the wafer (e.g., has a diameter of about 300 mm whenused with a 300 mm wafer) and resides in close proximity of the wafer,e.g., just below the wafer in a wafer-facing-down electroplatingapparatus. In some embodiments, the disk is relatively thin, for examplebetween about 5 and 50 mm thick. The plating electrolyte containedwithin the pores of the HRVA allows ionic current to pass though thedisk, but at a significant voltage drop compared to the system as awhole. For example, the voltage drop in the HRVA may be greater thanabout 50%, for example, between about 55 and 95%, of the total voltagedrop between the counter electrode (anode) and the wafer peripheraledge. In certain embodiments, the plated surface of the wafer resideswithin about 10 mm, and in some embodiments, within about 5 mm, of theclosest HRVA surface.

Further, in some embodiments the apparatus includes a secondary cathode,which is typically located at the periphery of the wafer substrate(e.g., having no footprint projected to the wafer). This secondarycathode, also referred to as a thieving cathode is negatively biasedduring at least a portion of electroplating and is configured to divertat least a portion of ionic current from the wafer periphery, therebyreducing plated thickness at the very edge of the wafer.

The apparatus will further include one or more power supplies and acontroller in association with the power supplies and with the elementsof the apparatus, wherein the controller is configured to perform themethods described herein. For example the controller may includeinstructions (e.g., in the form of program instructions or pre-builtlogic blocks) to specify electrical characteristics (e.g., current,voltage, power, polarity) provided to one or more components selectedfrom the group consisting of the wafer substrate, the auxiliaryelectrode, an anode, and the secondary (thief electrode). Theinstructions may be provided in some embodiments, by time-characteristicsequences (e.g., time-current sequences) for each of the elements.

In some embodiments, the main anode is located in a separated anodechamber, while the wafer substrate is located in a cathode chamber,wherein the two chambers are separated by an ion-permeable membrane(e.g., a Nafion® membrane). The compositions of electrolyte in the anodeand cathode chambers may be different. For example, catholyte in thecathode chamber may contain organic plating additives, while the anolyteis free of organic additives. In one configuration, the ionic currentcollimator and the auxiliary electrode are located in the separatedanode chamber.

In some embodiments, the auxiliary electrode is also separated from theanode by a cationic membrane such as a Nafion® membrane, while remainingin ionic communication with the electrolyte (e.g. anolyte). For example,the auxiliary electrode may reside in a chamber defined by the walls ofthe current collimator, the walls of the plating vessel, and thecationic membrane. The membrane preferably does not allow particulatematerial, which may form at the electrode due to flaking, to travelacross the membrane. The use of a membrane to isolate the auxiliaryelectrode can lead to electroplating with fewer defects.

In another aspect, a method for electroplating a layer of metal on awafer substrate is provided. In some embodiments, the method involves(a) providing the wafer substrate to an electroplating apparatus havinga wafer holder, and a plating vessel containing a main anode, anauxiliary electrode, and an ionic current collimator, wherein the ioniccurrent collimator is configured to direct ionic current from theperiphery to the center of the plating vessel; (b) in a firstelectroplating stage, electroplating metal onto the wafer substratewhile cathodically biasing the auxiliary electrode; and (c) in a secondelectroplating stage, electroplating metal onto the wafer substratewhile anodically biasing the auxiliary electrode.

Electroplating is performed by making one or more electrical contacts atthe periphery of the wafer substrate, wherein the contacts are made tothe conductive seed layer residing on the wafer substrate and bynegatively biasing the wafer substrate, so that it serves as a maincathode. In the beginning of the plating, the seed layer on thesubstrate is highly resistive and initially a relatively large currentshould be applied to a negatively biased auxiliary electrode. Typically,the cathodic current initially applied to the auxiliary electrode is atleast about 200%, such as at least about 300%, more preferably at leastabout 500%, such as between about 400-600% of the cathodic currentapplied to the wafer substrate. For example, when between about 10-15 Acurrent is applied to the wafer, between about 50-75 A is appliedinitially to the auxiliary electrode (functioning as a cathode). In someembodiments, the cathodic current initially applied to the auxiliaryelectrode is between about 10-75 A, such a s between about 20-50 A. Asplating proceeds and the terminal effect subsides, the cathodic currentapplied to the auxiliary electrode is reduced, in some embodiments.Reduction of cathodic current to the auxiliary electrode can follow anumber of current vs. time functions, e.g., over time the current can bereduced linearly, exponentially, or follow a polynomial function. Afterthe reduction, the auxiliary electrode is biased positively and startsserving as an auxiliary anode. In some embodiments the current suppliedto the auxiliary electrode (now in anode mode) is increased over time.In some embodiments the auxiliary electrode at least during a portion ofelectroplating time receives more anodic current than the main anode,thereby essentially serving as the main anode in the system. In someembodiments, when the auxiliary electrode is anodically biased, the mainanode (which was anodically biased in the beginning of plating), isswitched to being biased cathodically and remains cathodically biased atleast for a portion of electroplating. In some embodiments, in order tocompensate for the very strong terminal effect in the beginning of theplating, the plating tool may be configured to favor plating in thecenter of the wafer at the beginning of electroplating. For example, ina plating apparatus the electrical path from the anode to the edge ofthe wafer is configured to be more resistive than the electrical path tothe center of the wafer. In some embodiments, as plating proceeds, andterminal effect diminishes, electroplating rate in the center of thewafer can become too fast thereby having the potential to cause currentdensity non-uniformity across the wafer. In such cases, the currentprovided by the auxiliary electrode (which acts as an anode in thesecond stage of electroplating) will be increased with time and thecurrent provided by the main anode will be decreased with time. In somecases, where the plating apparatus provides particularly large amountsof plating current at the center of the wafer, the main anode isswitched to be biased cathodically in order to help further reduce thecenter-thick plating and to keep uniform current density distributionacross the whole wafer. In such cases, the main anode acts as acentrally based secondary cathode during a portion of electroplating,e.g., during at least a portion of the second stage of electroplating.

In some embodiments, the methods provided herein involve moving theionic current collimator along an axis that is perpendicular to theplane of the wafer substrate during electroplating. In some embodiments,the methods provided herein involve moving the auxiliary electrode alongan axis that is perpendicular to the plane of the wafer substrate duringelectroplating. The ionic current collimator and the auxiliary electrodein some embodiments are moved together in one block. In otherembodiments, the auxiliary electrode is moved separately from thecollimator.

In some embodiments, in addition to the ionic current collimator and theauxiliary electrode, the electroplating apparatus further includesadditional elements that are useful for mitigating the terminal effect.In some embodiments, an ionically resistive ionically permeable member,also known as HRVA resides between the anode and the wafer substrate.The auxiliary electrode preferably resides between the HRVA and theanode in the plating chamber. In some embodiments, the apparatus furtherincludes a secondary cathode, which is typically located at theperiphery of the wafer substrate (e.g., having no footprint projected tothe wafer). This secondary cathode, also referred to as a thievingcathode is negatively biased during at least a portion of electroplatingand is configured to divert at least a portion of ionic current from thewafer periphery, thereby reducing plated thickness at the very edge ofthe wafer. In some embodiments provided methods comprise providing acathodic current to the secondary cathode, wherein the current isbetween about 100-400%, more preferably is between about 200-300% of thecurrent provided to the wafer substrate at the beginning ofelectroplating. As electroplating proceeds, the current supplied to thesecondary cathode may be reduced, e.g., to zero, or to a small constantcurrent.

The use of a flexible auxiliary electrode, where the flexibility refersto its ability to function both as a cathode and an anode, and to beable to follow a number of current-time routines as desired by the user,allows for efficient control of ionic current distribution during theentire course of electroplating. Thereby, uniformly plated metal layerscan be obtained even when highly resistive seed layers are used or whenlarge wafers (e.g., 450 mm wafers) are used. The auxiliary electrode andthe ionic current collimator work in synergy to mitigate terminaleffect. The collimator directs the ionic current from the peripherygenerally in the central direction, thereby reducing the terminaleffect. In the presence of the collimator a relatively smaller currentcan be provided to the auxiliary electrode (in a cathode mode) at thebeginning of electroplating, to achieve mitigation of terminal effect.In addition, in some embodiments, the ionic current collimator providesan efficient platform in the plating chamber for a large auxiliaryelectrode.

The apparatus and process described hereinabove may be used inconjunction with lithographic patterning tools or processes, forexample, for the fabrication or manufacture of semiconductor devices,displays, LEDs, photovoltaic panels and the like. Typically, though notnecessarily, such tools/processes will be used or conducted together ina common fabrication facility. Lithographic patterning of a filmtypically comprises some or all of the following steps, each stepenabled with a number of possible tools: (1) application of photoresiston a workpiece, i.e., substrate, using a spin-on or spray-on tool; (2)curing of photoresist using a hot plate or furnace or UV curing tool;(3) exposing the photoresist to visible or UV or x-ray light with a toolsuch as a wafer stepper; (4) developing the resist so as to selectivelyremove resist and thereby pattern it using a tool such as a wet bench;(5) transferring the resist pattern into an underlying film or workpieceby using a dry or plasma-assisted etching tool; and (6) removing theresist using a tool such as an RF or microwave plasma resist stripper.In some embodiments, the electroplating methods provided herein furtherinclude lithographic steps of: applying photoresist to the workpiece;exposing the photoresist to light; patterning the resist andtransferring the pattern to the workpiece; and selectively removing thephotoresist from the work piece. In some embodiments, a system isprovided which includes the electrodeposition apparatus described hereinand a stepper.

Another aspect of the invention is an apparatus configured to accomplishthe methods described herein. A suitable apparatus includes hardware foraccomplishing the process operations and a system controller havinginstructions for controlling process operations in accordance with thepresent invention. The system controller will typically include one ormore memory devices and one or more processors configured to execute theinstructions so that the apparatus will perform a method in accordancewith the present invention. Machine-readable media containinginstructions for controlling process operations in accordance with thepresent invention may be coupled to the system controller. In someembodiments, an apparatus is provided, wherein the apparatus comprises aplating vessel, a wafer holder, an auxiliary electrode, an ioniccollimator and a controller comprising program instructions and/or builtin logic for (a) in a first electroplating stage, electroplating metalonto the wafer substrate while cathodically biasing the auxiliaryelectrode; and (b) in a second electroplating stage, electroplatingmetal onto the wafer substrate while anodically biasing the auxiliaryelectrode.

Advanced technologies call for the electroplating of metals onto waferswith sheet resistances of 10 ohm per square and higher (even 20 ohms persquare or 40 ohms per square or higher). As seed layers gets thinner,and as wafer sizes get bigger, the difference in plating thicknessbetween the center and edge (thus terminal effect) becomes morepronounced. This requires ever more aggressive techniques to compensatefor the terminal effect. During plating, the thickness of metal and thesheet resistance can drop several orders of magnitude in a short time,and so methods and apparatus capable of plating uniformly on the waferthroughout a process where there may be a rapidly initially varying andlater a relatively constant sheet resistance are required. Embodimentsof the present invention address the challenges presented by such highresistance seed layers, the rapid dynamic variance in the seedelectrical parameters, and the extreme terminal effect they present.

Embodiments of the present invention pertain to methods and apparatusesfor electroplating a substantially uniform layer of metal onto a workpiece having a seed layer thereon. In certain embodiments, a platingcell includes both an ionic current collimator positioned in theproximity of an anode, and an auxiliary electrode configured to functionas an auxiliary cathode in the beginning of electroplating and as anauxiliary anode at later stages of electroplating. The describedconfiguration, particularly when used in combination with a HRVA and asecondary cathode, can maintain uniform current distribution throughoutplating process. In some cases, however, it may be desirable to useembodiments of the invention to create a non-uniform current densitythat is experienced by the wafer. For example, it may be desirable tocreate a non-uniform current density, resulting in non-uniform metalplating, during overburden deposition to aid in chemical mechanicalpolishing (CMP), wet chemical etching, electropolishing, orelectromechanical polishing.

A cross-sectional schematic view of an example of an electroplatingapparatus having an ionic current collimator and a flexible auxiliaryelectrode is shown in FIG. 1A. Electrical connectivity is not shown topreserve clarity

The apparatus depicted in FIG. 1A includes a plating vessel 101, whichis configured to hold electrolyte 102 (e.g. an aqueous solution of acopper salt, an acid, and electroplating additives) in contact with thewafer substrate 103 during electroplating. The wafer substrate 103 isheld in a face-down orientation by the wafer holder 104. The waferholder includes electrical contacts that are configured to contact thewafer 103 at its periphery (but not in the center), which areelectrically connected to a power supply (not shown). In someembodiments, the wafer holder 104 is a clamshell apparatus which makescontacts to the periphery of the wafer through a number of contactfingers housed behind a typically elastic “lip seal”, which serves toseal the clamshell and keep the edge contact region and wafer backsidesubstantially free of electrolyte, as well as to avoid any plating ontothe contacts. A general description of a clamshell-type platingapparatus having aspects suitable for use with this invention isdescribed in detail in U.S. Pat. No. 6,156,167 issued to Patton et al.,and U.S. Pat. No. 6,800,187 issued to Reid et al, which are bothincorporated herein by reference for all purposes. The wafer holder isalso configured to rotate the wafer substrate during electroplating.

The apparatus is configured to negatively bias the wafer substrateduring electroplating such that it serves as a cathode. Theelectroplating vessel 101 further includes an anode 105, located at adistance below the wafer substrate. The anode is electrically connectedto a power supply and is configured to be positively biased relative tothe wafer substrate during plating (e.g., it can be held at a groundpotential). An active anode (e.g. copper-containing anode) is typicallyused. For applications used for deposition of other metals either anactive anode or an inert anode could be used. It is noted that in someembodiments, the main anode 105 may also be configured to serve as acathode during some stages of electroplating (e.g., when the auxiliaryelectrode assumes an anodic function). In these embodiments, the mainanode is connected with one or more power supplies that are capable ofbiasing it both anodically and cathodically.

In the depicted embodiment, the apparatus includes an anode chamber 107and a cathode chamber 109, which are separated by an ion-permeablemembrane 111. A cationic membrane, such as Nafion may be used as amembrane 111. The composition of electrolyte in the anode chamber 107and in the cathode chamber 111 may be the same or different. In someembodiments, the electrolyte in the anode chamber (anolyte) containsions of plateable metal, but no organic plating additives, while theelectrolyte in the cathode chamber (catholyte) contains both ions ofplateable metal and organic plating additives (e.g., one or more ofaccelerators, suppressors, and levelers). The cationic membrane 111allows ionic communication between the separated anolyte chamber and thecathode chamber, while preventing the particles generated at the anodefrom entering the proximity of the wafer and contaminating it. Thecationic membrane is also useful in prohibiting non-ionic and anionicspecies such as bath additives from passing though the membrane andbeing degraded at the anode surface, and to a lesser extent inredistributing current flow during the plating process and therebyimproving the plating uniformity. Detailed descriptions of suitableionic membranes are provided in U.S. Pat. Nos. 6,126,798 and 6,569,299issued to Reid et al., both incorporated herein by reference. A detaileddescription of suitable cationic membranes is provided in U.S. patentapplication Ser. No. 12/337,147, entitled Electroplating Apparatus withVented Electrolyte Manifold, filed Dec. 17, 2008, incorporated herein byreference. Further detailed description of suitable cationic membranesis provided in U.S. patent application Ser. No. 61/139,178, entitledPlating Method and Apparatus with Multiple Internally IrrigatedChambers, filed Dec. 19, 2008, incorporated herein by reference.

An ionically resistive ionically permeable element 113 (also known asHRVA) is located directly below the wafer 103 and resides in the cathodechamber 109. HRVA, in some embodiments, is a plate made of a dielectricmaterial, which has a plurality of non-communicating holes, whichprovide a resistive path for ionic current in the proximity of thesubstrate. HRVA is described in detail in the US Patent Pub. No.2010/0116672 by Mayer et al. titled Method and Apparatus forElectroplating, filed Jun. 9, 2009, incorporated herein by reference.

A thieving secondary cathode 115 resides in this embodiment in its ownchamber filled with electrolyte and is in ionic communication withelectrolyte in the cathode chamber. The secondary cathode 115 iselectrically connected to a power supply and is configured to benegatively biased at least during a portion of electroplating. Theopening of the chamber, which defines the virtual thief cathode islocated between the HRVA 113 and the wafer 103, referring to position onthe vertical axis. This location allows for efficient diversion of ioniccurrent from the near edge region of the wafer. A thief cathode (bothphysical and virtual) in this location is described in detail in theU.S. patent application Ser. No. 12/481,503, previously incorporated byreference. In some embodiments the thief cathode is a metal ringperipherally located relative to the wafer substrate.

Referring again to FIG. 1, the anode chamber 107 houses the ioniccurrent collimator 117 and an auxiliary electrode 119. The ionic currentcollimator 117 is located above the anode 105. The current collimatorhas a current restricting portion generally parallel to the plane of theanode, and attached to the walls of the plating chamber, and acylindrical central portion which is generally in the form of an opencylinder extending in the direction that is perpendicular to the planeof the wafer substrate, and, typically, co-centered with the center ofthe wafer substrate and the center of the anode. The openings of thecylinder provide a route for the ionic current to move from the anodeupward in the direction of the wafer substrate. The current collimatoras a whole directs the current from the anode in the direction ofcentral portions of the wafer, and thereby provides a resistivecompensation for the terminal effect. However, this compensation aloneis not sufficient for plating on highly resistive seed layers.Accordingly, an auxiliary electrode configured to redistribute thecurrent from the opening of the collimator, away from the center of thewafer is needed.

The auxiliary electrode 119 in the depicted embodiment resides on top ofthe current-restricting portion of the ionic current collimator 117. Theauxiliary electrode 119 is electrically connected to a power supply andis configured to be biased both cathodically and anodically during thecourse of electroplating of a single substrate. In some embodiments, theauxiliary electrode is negatively biased in the beginning ofelectroplating, when the terminal effect is pronounced, and later isanodically biased. Because the auxiliary electrode serves as an anode,in some embodiments it is made of a material that is plated on the wafersubstrate, e.g., copper during copper plating. In other embodiments, ithas a coating of a metal that is being plated on the substrate and acore made of a different metal. Yet in other embodiments the auxiliaryelectrode may be made of a material that is different from the one beingplated, but it is sufficiently coated with the plated metal (e.g.,copper) during the period when it serves as a cathode. This depositedmaterial is then redissolved when the auxiliary electrode serves as ananode.

A cross-sectional schematic view of another example of an electroplatingapparatus having an ionic current collimator and an auxiliary electrodeis shown in FIG. 1B. In this apparatus, in addition to the cationicmembrane 111 residing below the HRVA, a second cationic membrane 111 ais added, such that the membrane 111 a resides directly above theauxiliary electrode 119, so that a chamber 120 is formed. The auxiliaryelectrode chamber 120 is defined by the sidewall of the apparatus on oneside, by the ionic current collimator 117 on the bottom and on the otherside and by the cationic membrane 111 a on top. The cationic membrane111 a separates the auxiliary electrode from the anode, such that anyparticles generated at the auxiliary electrode, would not be able tocross the membrane. The membrane, however, allows for ioniccommunication between the auxiliary electrode chamber and anolyte, asthe cationic membrane allows for transfer of cations. The cationicmembrane 111 a is attached to the sidewalls of the plating vessel, andis typically also attached to the opening of the cylinder of the ioniccurrent collimator 117 (e.g., by an o-ring). The auxiliary electrode maybe prone to flaking, due to plating and deplating cycles, and thereforeits isolation via a cationic membrane is preferred in some embodiments.

A cross-sectional schematic view of yet another example of anelectroplating apparatus having an ionic current collimator and aflexible auxiliary electrode is shown in FIG. 1C. In thisimplementation, the cationic membrane 111 a isolating the auxiliaryelectrode 119 in a chamber 120 is present, but the cationic membrane 111in the proximity of the HRVA is absent.

The relative positions of the ionic current collimator and of theauxiliary electrode are significant characteristics of the describedapparatus. The ionic current collimator and the auxiliary electrode workin synergy to provide a flexible solution for modulating ionic currentdistribution and, consequently, for plating uniformity during the courseof electroplating under changing conditions.

The current collimator is described in more detail with reference toFIGS. 2A and 2B. FIG. 2A shows a diagrammatical cross-sectional view ofthe ionic current collimator 217 residing over the anode 205. Thecentral portion 221 of the ionic current collimator 217 is a hollowcylinder having a diameter d1 and height d2. In some embodiments thediameter d1 is between about 30-70% of the wafer radius, more preferablybetween about 40-60% of the wafer radius, such as between about 90-135mm. The height d2 can be between about 30-60% of d1, in someembodiments. The central portion of the ionic current collimator isattached to the current restricting portion 223, where the currentrestricting portion 223 is parallel to the anode. The currentrestricting portion extends to the walls of the plating vessel and has alength d3 of between about 30-70% of the wafer radius. The ionic currentcollimator is positioned such that it restricts the ionic current fromescaping at the periphery and directs substantially all current from theanode to the opening of the cylinder in the central portion 221. The topsurface of the collimator and the HRVA bottom surface are maintained ata non-zero distance to allow for current redistribution to the auxiliaryelectrode. In some embodiment this distance is less than half of thewafer radius.

In some embodiments the ionic current collimator does not have twodistinct portions described above, but simply has an ion restrictingportion with an opening in the center. The ionic current collimator inthese embodiments may have a toroidal shape with a uniform thickness(along an axis perpendicular to the wafer surface). However, anembodiment of collimator with a central cylindrical portion extendingupward towards the wafer has a number of advantages over a simplerdoughnut-shaped collimator. For example, the collimator having a centralcylindrical portion extending upward is typically more effective indelivering the ionic current to the central portions of the wafer, andalso serves as a more convenient platform for the auxiliary electrode.

FIG. 2B illustrates a top view of the ionic current collimator 217,showing an extended disk of the current restricting portion 223 aroundthe opening of the central portion 221.

FIG. 3A illustrates a cross-sectional schematic view of a portion of theelectroplating apparatus which includes the anode 305, the currentcollimator 317, and the auxiliary electrode 319 which resides on thecurrent restricting portion of the current collimator. The auxiliaryelectrode 319 has a toroidal shape and is located at least partiallyover the anode (has a non-zero footprint when projected onto the anode).The footprint projected onto the anode is shown as d5. In someembodiments, its footprint projected onto the anode is at least about40% of total anode area, e.g., between about 40 and 80% of the totalanode area. In some embodiments, the footprint of the currentrestricting portion of the ionic current collimator onto the anode isvery similar to the footprint of the auxiliary anode, e.g., at leastabout 40% of total anode area, e.g., between about 40 and 80%.

Further, in some embodiments the auxiliary electrode has a large surfacearea. When the surface area is large, the auxiliary electrode can accepta very high current (as often needed to compensate for the terminaleffect caused by highly resistive seed layers), without buildingundesirably high current densities. In some embodiments, the surfacearea of the auxiliary electrode is at least about 600 cm², such asbetween about 900 cm² and 1200 cm². FIG. 3B illustrates a top view ofthe toroidal auxiliary electrode 319. In some embodiments, the electrodehas a radial thickness d6 (difference between outer and inner radius)that is at least 60 mm, such as between about 60 mm and 150 mm.

FIG. 4 is a schematic illustration of electrical connectivity betweenthe elements of the electroplating apparatus in accordance with oneembodiment. The wafer substrate 403, the anode 405, the secondary thiefcathode 415 and the auxiliary electrode 419 are connected to one or morepower supplies 431 (shown as one block), which are configured to biasthe substrate 403 negatively during electroplating, while concurrentlybiasing the anode 405 positively relative to the substrate, and whilebiasing the thief cathode 415 negatively at least during a portion ofelectroplating of one substrate, and while biasing the auxiliaryelectrode both negatively and positively during the course ofelectroplating of one substrate. Typically, the auxiliary electrode isnegatively biased in the beginning of electroplating, and is positivelybiased later in the plating process. In some embodiments, the one ormore power supplies 431 are also configured to bias the main anodecathodically, when the auxiliary electrode is biased anodically.

In certain embodiments, one or more power supplies are provided forproviding power to the wafer substrate, the auxiliary electrode, and thesecondary cathode. In some cases, a separate power supply is providedfor each of the auxiliary electrode, the secondary cathode and the workpiece; this allows flexible and independent control over delivery ofpower to each cathode. Alternatively, one power supply with multipleindependently controllable electrical outlets can be used to providedifferent levels of current to the wafer, to the auxiliary electrode,and to the secondary cathode. In the embodiment depicted in FIG. 4, theanode is positively biased with respect to the wafer, the auxiliaryelectrode, and the secondary cathode, and is sometimes grounded.

In some embodiments the power supply 413 is a multi-channel powersupply. One channel of the power supply 431 negatively biases the wafer403 with respect to the anode; another channel of the power supplynegatively biases the secondary cathode 415 with respect to the anode;and another channel of the power supply 431 negatively biases theauxiliary electrode in the beginning of plating with respect to theanode. The power supply 431 can be connected to a controller 433, whichallows for independent control of current and potential provided to thewafer, the secondary cathode and auxiliary electrode of theelectroplating apparatus. Power supply 431 causes an electrical currentto flow from anode 405 to wafer 403, plating metal onto the wafer. Thechannel connected to secondary cathode 415 of power supply 431 causesthe electrical current to flow from anode 405 to the secondary cathode415, thereby partially or substantially diverting current that flowsfrom anode 405 to wafer 403. Depending on the relative potential appliedon the auxiliary electrode 419, auxiliary electrode 419 may either pullcurrent from anode 405, or provide current to wafer 403. The electricalcircuit described above may also include one or several diodes (notshown) that will prevent reversal of the current flow, when suchreversal is not desired.

With separate power supplies or power supply channels for the auxiliaryelectrode, the secondary cathode, and the wafer, the current applied toeach of the electrodes may be dynamically controlled. As the wafer iselectroplated with metal, the sheet resistance decreases and the currentdistribution non-uniformity may be reduced, making the secondary cathodeunnecessary after a certain thickness of metal is achieved. The currentsupplied to the secondary cathode and the auxiliary electrode may bedynamically controlled to account for a reduction of the wafer's sheetresistance and for the associated more uniform current distribution thatnormally results without the activation of the auxiliary electrode. Insome embodiments, no current is supplied to the secondary cathode afterthe sheet resistance of the wafer drops to a defined level such as about1 ohm per square and less. In some embodiments, the current that issupplied to the auxiliary electrode (in cathode mode) changes polarityafter the sheet resistance of the wafer drops to a defined level such asabout 7.5 ohm per square or lower, after which point the auxiliarycathode becomes an anode and starts to supply current to wafer 403. Inother words, in one process sequence, in the very beginning of plating,both the secondary cathode (thief) and the auxiliary electrode (incathode mode) are powered, and typically each receives more current thanthe wafer. Next, after a certain amount of plating, the polarity of theauxiliary electrode is switched from negative to positive, and then,after a further amount of plating, power to the thief is turned off,while the wafer is negatively biased all throughout the processreceiving constant or changing current.

A controller 433 is electrically connected with the one or more powersupplies 431 and is configured to control the electrical parametersapplied to the apparatus components. For example the controller cancontrol one or more of power, current and voltage applied to thecomponents of the apparatus, and is capable of dynamically changingthese parameters during the course of electroplating. In someembodiments, the controller includes program instructions, or logic forperforming the methods provided herein.

An electroplating method, in accordance with one embodiment, isillustrated by the process diagram shown in FIG. 5A. The process startsin 501 by placing a wafer having a continuous seed or barrier-seed layerin a wafer holder of an electroplating apparatus having an ionic currentcollimator configured to direct ionic current from the anode to thecentral portions of the wafer. Next, in 503, the wafer (in contact withelectrolyte) is biased negatively to plate metal on the seed orbarrier-seed layer, while concurrently a thief cathode located at theperiphery of the wafer is negatively biased to divert ionic current fromthe near-edge region of the wafer. Concurrently, an auxiliary electrodelocated between the anode and the wafer is negatively biased. Next, in505, the current supplied to the auxiliary electrode is reduced, andnext, the auxiliary electrode is biased positively. Further, the currentsupplied to the thief cathode is reduced to a small amount, or the powerto the thief cathode is turned off. Further, the current supplied to themain anode is reduced during the course of plating or even reversed tocathodic current, in some embodiments. The metal is plated in 507 untila desired thickness is reached.

In some embodiments the cathodic current provided to the auxiliaryelectrode in the beginning of electroplating is at least 200% of thecurrent provided to the wafer substrate, such as at least 400% of thecurrent provided to the wafer substrate. In some embodiments, thiscurrent is rapidly reduced, e.g., in the first 5 seconds ofelectroplating time, and is then switched to anodic current, which maybe increased during the course of electroplating. In some embodiments,the current is decreased following an exponential function. In otherembodiments, the current is decreased following a polynomial function.The time of the switch to the anodic mode can be determined, bycalculating the projected sheet resistance of the wafer based on anumber of input parameters (e.g., type of metal plated, size of wafer,and plating speed). Alternatively the projected sheet resistance may becalculated by measuring the number of coulombs that pass through thesystem, and knowing the type of metal being plated, and the size of thewafer. The switch time may occur after a certain projected sheetresistance has been reached. For example, for a deposition of a knownmetal on a wafer of a known size, it can be calculated at what time thesheet resistance decreases below a predetermined number. Therefore, theauxiliary electrode can be switched to an anode mode when the sheetresistance drops below a predetermined number (e.g., below 7.5 ohm/sq.)

FIG. 5B illustrates an exemplary algorithm for configuring a systemcontroller to perform the methods provided herein. In operation 511 aplurality of input parameters are provided. These input parameters mayinclude the size of the wafer, the type of metal, and the plating speed.Next, in 513, current-time profiles for one or more of the wafer, thesecondary thief cathode, and the auxiliary electrode are calculatedbased on the input parameters. In 517, the controller having thesecurrent-time instructions is interfaced with the one or more powersupplies and with the components of the electroplating system. Next, in519, the metal is plated following the instructions provided by thesystem controller.

Controller 433 in conjunction with power supplies 431 allows forindependent control of current and potential provided to the wafer, theauxiliary cathode, and the second auxiliary cathode of theelectroplating apparatus, as well as control over other platingcomponents. Thus, controller 433 is capable of controlling powersupplies 431 to generate the current profiles described above. Thecontroller, however, generally is not capable of independentlydetermining if one of the conditions described above (e.g., sheetresistance reaching a level of 1 ohm per square or lower) has been met,though an estimate of the sheet resistance can be made based on a knowntotal cumulative amount of charge passed to the wafer at any given time.Thus, the controller may be used in conjunction with sensors that maydetermine whether a condition has been met. Alternatively, thecontroller may be programmed with a separate current versus time profilefor each of the wafer, auxiliary electrode, and the secondary cathode.The controller may also measure the charge (coulombs=integral ofamperage*time) supplied to the wafer, auxiliary electrode, and secondarycathode, and base the current-time profile on these data.

Controller 433 may be configured to control electrical power deliveredto the auxiliary electrode in a manner that produces a more uniformcurrent distribution from the anode after electroplating a definedamount of metal onto the substrate or after electroplating for a definedperiod of time. Controller 433 may also be configured to controlelectrical power delivered to a secondary cathode adapted for divertinga portion of ionic current from an edge region of the substrate.Furthermore, controller 433 may be configured to ramp down electricalpower delivered to the auxiliary electrode 419 and the secondary cathode415, each at different rates, as metal is deposited on the substrate.Additionally, controller 433 may be configured to supply anodic currentafter supplying cathodic current to the auxiliary electrode after thesheet resistance of the substrate surface reaches a first thresholdlevel, and to supply no current or substantially no current to thesecondary cathode after the sheet resistance of the substrate surfacereaches a second threshold level.

Controller 433 may be further designed or configured to control theposition of the wafer relative to the anode position, the rotation ofthe wafer in the wafer holder, etc. In the case where the auxiliaryelectrode and/or the collimator is movable, controller 433 can alsocontrol the movement parameters of the collimator and/or of theauxiliary cathode, such as the speed of movement and timing of startingand stopping the movement. The positions of the collimator and of theauxiliary cathode may be controlled based on a number of factorsincluding, but not limited to, the sheet resistance of the substratesurface, time (i.e. how long the electrodeposition process has beengoing), and the amount of metal deposited onto the substrate surface.These factors allow for dynamic control of the collimator position,resulting in more uniform deposition across the wafer. For example, thecontroller may be configured and may comprise program instructions toinitiate movement of one or both of the collimator and of the auxiliaryelectrode after a threshold sheet resistance is reached, or after apredetermined amount of time, or after a predetermined amount of platinghas occurred.

FIG. 6A illustrates results of computational modeling illustrating ioniccurrent distribution in the beginning of electroplating, when theauxiliary electrode serves as an auxiliary cathode. In the illustratedsystem, the ionic current is directed from the main anode 605 (at aground potential) to the central opening of the ionic current collimator617, from where it proceeds to the central portions of the wafer 603(negatively biased), and is partially diverted towards the negativelybiased auxiliary electrode 619. The excess of ionic current reaching theedges of the wafer 603 is diverted to a negatively biased thief cathode615 which resides in a chamber around the periphery of the wafer and isconnected to the main plating bath via a narrow channel.

FIG. 6B illustrates results of computational modeling illustrating ioniccurrent distribution at a later stage in electroplating. In this casepower to the secondary thief cathode 615 is turned off, and theauxiliary electrode 619 is positively biased and serves as an anode. Themain anode 605 remains at a ground potential. It can be seen that theionic current is now supplied both by the anode 605 and by the auxiliaryelectrode 619, where the ionic current from 619 is directed primarily tonon-central regions of the wafer substrate 603. Thus, in a first stageof electroplating the system contains one anode, and three cathodes,while in the later stage of electroplating the system contains twoanodes and one cathode (wafer).

FIG. 7 illustrates a plot of optimal instantaneous currents on thecomponents of the plating cell for seed layers having different sheetresistances, in accordance with one implementation. The range of sheetresistances is from about 0.05 ohm/sq (corresponding to about 4000 Åcopper layer) to about 50 ohm/sq. The curve (a) shows that the cathodiccurrent provided to the wafer substrate is constant at 10 A. Curve (b)illustrates the cathodic current provided to the secondary cathodeduring plating. As the sheet resistances decrease, the amount ofcathodic current supplied to the secondary cathode also decreases. Curve(c) illustrates the current supplied to the auxiliary electrode. Forhighly resistive seed layers (7.5-50 Ohm/sq) a cathodic current issupplied to this electrode. The cathodic current decreases as the sheetresistance decreases from 50 Ohm/sq. to 7.5 Ohm/sq. At 7.5 Ohm/square nocurrent is supplied to this electrode, and at lower sheet resistances,the polarity of the electrode is switched, and it starts acceptingpositive current and starts serving as an auxiliary anode. The anodiccurrent is increased, as the sheet resistance is further decreased.Curve (d) shows the current at the main anode. The current at the anodedecreases as the sheet resistances decrease (anodic current for the mainanode is defined as positive in this plot).

FIG. 8 illustrates one exemplary scenario of current vs. time profilethat may be employed for the auxiliary electrode when plating onresistive seeds. The process starts by applying a cathodic current ofabout 50 A to the auxiliary cathode, rapidly decreasing the cathodiccurrent over less than 5 seconds, then transitioning to anodic current,then increasing the anodic current, and then plating at a relativelyconstant anodic current at the auxiliary electrode for at least 30seconds. The current to the wafer follows the following waveform: 10Afor the first 5 seconds, followed by 15 A for the next 30 seconds,followed by 90 A for the remainder of the plating time. For the purposesof this calculation the current is normalized to a constant wafercurrent of 10 A. The current to the main anode decreased over time, suchthat after about 30 seconds of plating, the anodic current of theauxiliary electrode was higher than at the “main” anode. Therefore, theanodes switched roles in this scenario, with the auxiliary anode servingas the “main” anode during the later portion of plating.

FIG. 9 illustrates computational modeling results showing distributionof current at the surface of the wafer during one exemplaryelectroplating sequence. The X-axis refers to a radial position on thewafer, starting at a center (0 m) and extending to the edge (0.225 m).The Y-axis shows the level of current in Amp/m². The electroplatingprocess begins in (a), where the sheet resistance of the seed layer is50 ohm.sq., the wafer receives a cathodic current of 10 A, the secondarythief cathode receives a cathodic current of 27.7 A, and the auxiliaryelectrode receives a cathodic current of 49.7 A. Next, after 0.45seconds in (b) the sheet resistance drops to 30 Ohm/sq., the waferreceives a cathodic current of 10 A, the secondary thief cathodereceives a smaller cathodic current of 16.8 A, and the auxiliaryelectrode receives a smaller cathodic current of 24.6 A. Next, after 2.1seconds after beginning of plating, in (c) the sheet resistance dropsfurther to 10 Ohm/sq., the wafer receives a cathodic current of 10 A,the secondary thief cathode receives a smaller cathodic current of 5.6A, and the auxiliary electrode receives a smaller cathodic current of2.7 A. Next, after 11 seconds after beginning of plating, in (d) thesheet resistance drops further to 1 Ohm/sq., the wafer receives acathodic current of 10 A, the secondary thief cathode receives a smallercathodic current of 0.1 A, and the auxiliary electrode receives switchespolarity and receives an anodic current of −5 A. Finally, after 45seconds after beginning of plating, in (e) the sheet resistance dropsfurther to 0.05 Ohm/sq., the wafer receives a cathodic current of 10 A,the secondary thief cathode is turned off and receives zero current, andthe auxiliary electrode receives a higher anodic current of −6.9 A. Itcan be seen that a highly uniform distribution of current across thewafer surface, and, consequently, uniform plating can be achieved usingprovided electroplating sequences.

The programming of the apparatus controller with a desired current-timecharacteristic can be performed, in some embodiments, following theexemplary guidelines presented below.

As shown in FIG. 7, the optimal current on the auxiliary electrode andon the secondary cathode is linearly correlated with the instantaneoussheet resistance of the seed wafer substrate. Thus in a plating process,due to constant plating occurring on the wafer substrate, the preferredprocess involves dynamic changing and smart control of current on theauxiliary electrode and on the secondary cathode.

Control of the current level on the auxiliary electrode (as well as onthe secondary cathode) is designed in some embodiments based on thelinear correlation between the optimal current level and theinstantaneous substrate sheet resistance. The following three aspectscan be merged into a single mathematical model which can be implementedthrough waveform control (current-time function for each of thecomponents of the apparatus). This function can be included in thecontroller in the form of program instructions, e.g., via inclusion intothe power supply firmware control unit. According to a first aspect,there is a linear correlation between the sheet resistance of the wafersubstrate and the optimal current on the auxiliary cathode. According toa second aspect, in a plating process, the metal growth rate (thus the“seed” thickness increase rate) follows a step function. Thus, over acertain time period the growth rate is constant. Therefore, thethickness of the metal on the wafer substrate is linearly correlatedwith the time over each time period. According to a third aspect, thecorrelation between the sheet resistance of the wafer substrate and thethickness of the “seed” layer is not linear. Instead it could bemathematically illustrated as a polynominal function or an exponentialfunction. The presented three correlations, when merged together, leadto a polynominal and/or exponential correlation between the optimalauxiliary electrode current and plating time, as illustrated in FIG. 8.Accordingly, in some embodiments, an instantaneous measurement of thesubstrate sheet resistance is not necessary.

In some embodiments, the control of the system is implemented using thefollowing steps without using the sensors. In a first step,computational modeling is performed to determine the optimal current onthe auxiliary electrode versus wafer sheet resistance for a givenhardware design, as illustrated in FIG. 7. Next, confirming experimentsare performed to finalize the determination of correlation betweenauxiliary electrode current and time for different metal layer growthrates. The obtained correlation will be suitable for providing controlfor a selected hardware design with certain hardware dimensions andarrangement. The obtained correlation can then be used to configurepower supply firmware and software and to pre-define the constants of apolynominal function and/or an exponential function used by thecontroller, and thus define the current-time function. Input parametersprovided by the apparatus user to configure the controller may include:initial seed layer thickness on the substrate, initial current on theauxiliary electrode, information that defines starting point ofpolynominal and/or exponential function, and initial plating current onthe wafer substrate, which is determined by factors other than theauxiliary electrode parameters. Factors that determine the platingcurrent on the wafer substrate include but are not limited to: types ofstructures on the wafer substrate, plating bath chemistry used forplating on the wafer substrate, integration requirements, etc.

CONCLUSION

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art.Although various details have been omitted for clarity's sake, variousdesign alternatives may be implemented. Therefore, the present examplesare to be considered as illustrative and not restrictive, and theinvention is not to be limited to the details given herein, but may bemodified within the scope of the appended claims. Further it isunderstood that many features presented in this application can bepracticed separately as well as in any suitable combination with eachother, as will be understood by one of skill in the art.

1. A method of electroplating a metal on a wafer substrate, the methodcomprising: (a) providing the wafer substrate to an electroplatingapparatus having a wafer holder, and a plating vessel containing a mainanode, an auxiliary electrode, and an ionic current collimator, whereinthe ionic current collimator is configured to direct ionic current fromthe periphery to the center of the plating vessel; (b) in a firstelectroplating stage, electroplating metal onto the wafer substratewhile cathodically biasing the auxiliary electrode; and (c) in a secondelectroplating stage, electroplating metal onto the wafer substratewhile anodically biasing the auxiliary electrode.
 2. The method of claim1, wherein the second electroplating stage is performed after the firstelectroplating stage, and wherein during the first electroplating stagea cathodic current initially applied to the auxiliary electrode is atleast about 300% of a cathodic current applied to the wafer substrate atthe same time.
 3. The method of claim 1, wherein in the secondelectroplating stage, the auxiliary electrode is anodically biased to agreater extent than the main anode at least during a portion of thesecond electroplating stage.
 4. The method of claim 1, furthercomprising cathodically biasing the main anode during at least a portionof the second electroplating stage.
 5. The method of claim 1, furthercomprising moving the ionic current collimator and the auxiliaryelectrode in a direction perpendicular to a plating surface of the wafersubstrate, during the electroplating.
 6. The method of claim 1, furthercomprising the steps of: applying photoresist to the wafer substrate;exposing the photoresist to light; patterning the photoresist andtransferring the pattern to the wafer substrate; and selectivelyremoving the photoresist from the wafer substrate.
 7. The method ofclaim 1, wherein the ionic current collimator comprises: (i) a centralportion in the form of an open cylinder extending in a direction that isperpendicular to a plating surface of the wafer substrate, wherein theopenings of the cylinder provide a route for the ionic current; and (ii)a current restricting portion connected to the central portion, thecurrent restricting portion extending in a direction that is parallel tothe plating surface of the wafer substrate.
 8. The method of claim 7,wherein the current restricting portion of the ionic current collimatorextends to sidewalls of the plating vessel, and is configured to blockionic current at the periphery of the plating vessel.
 9. The method ofclaim 8, wherein the current restricting portion of the ionic currentcollimator is attached to the sidewalls of the plating vessel.
 10. Themethod of claim 1, wherein the ionic current collimator is made of adielectric material that is not permeable to electrolyte and is selectedfrom the group consisting of polycarbonate, polyethylene, polypropylene,polyvinylidene difluoride (PVDF), polytetrafluoroethylene, andpolysulphone.
 11. The method of claim 1, wherein the ionic currentcollimator does not contact the main anode and is spaced from the mainanode by a distance of at least about 15% of the wafer substrate radius.12. The method of claim 1, wherein the auxiliary electrode resides overthe main anode and between the ionic current collimator and the wafersubstrate holder.
 13. The method of claim 1, wherein the ionic currentcollimator serves as a platform supporting the auxiliary electrode. 14.The method of claim 1, wherein the auxiliary electrode comprises copperat least on the surface of the auxiliary electrode.
 15. The method ofclaim 1, wherein a footprint of the auxiliary electrode onto the mainanode is at least about 40% of a total anode area.
 16. The method ofclaim 1, wherein the auxiliary electrode has a generally toroidal shapeand a thickness of at least about 20 mm.
 17. The method of claim 1,wherein the auxiliary electrode has a working surface of at least about600 cm².
 18. The method of claim 1, wherein the apparatus furtherincludes a thief cathode, which is cathodically biased during a portionof electroplating.
 19. The method of claim 1, wherein the apparatusfurther includes a thief cathode, which is cathodically biased duringthe first electroplating stage and during a portion of the secondelectroplating stage.
 20. The method of claim 1, wherein the auxiliaryelectrode changes polarity from cathodic to anodic after a defined sheetresistance of the electroplated layer on the wafer substrate is reached.